A Community Antenna Television ("CATV") system, or cable television system, includes at its basic elements a centralized transmitter and a distribution network. The centralized transmitter typically includes an aerial antenna for receiving radio frequency ("RF") television signals. The centralized transmitter is furthermore connected to communicate television signals to CATV subscribers through the distribution network. The distribution network includes miles of coaxial cable that often connect as many as hundreds of thousands subscribers to the centralized transmitter. The complexity and size of the distribution network require that network operation and performance be periodically tested and/or monitored.
Two tests often performed by CATV service providers are signal level monitoring and leakage detection. CATV service providers use signal level monitors to measure the signal level of particular channel frequencies at any part of the distribution network. A technician connects the signal level monitor to the coaxial cable at any location within the distribution network. The signal level monitor allows the technician to obtain data regarding the frequency response of the distribution network and identify network-related signal transmission problems.
Leakage detectors are devices that detect and/or measure the leakage of CATV signals to the exterior of the coaxial cable. If the coaxial cable is insufficiently shielded, significant levels of the CATV signals may leak to the environment surrounding the cable. Government regulations permit only a finite level of coaxial cable leakage. Leakage detectors help determine compliance with government regulations and can otherwise provide information as to the performance of particular sections of coaxial cable.
Preferably, CATV test equipment is designed to have relatively small physical size. Small physical size is preferable because the equipment must be portable as it is intended for use in various parts of the distribution network. Transportation costs are reduced and the convenience to the technician is increased when the size and bulk of the testing equipment are reduced. One way to reduce the overall bulk of test equipment is to reduce the actual number of test devices that technicians must have on hand. To reduce the quantity of test equipment needed by CATV test technicians, devices have been developed that perform multiple testing functions. For example, U.S. Pat. No. 5,493,210 to Orndorff, issued Feb. 20, 1996, filed Jun. 10, 1993, ("Orndorff") shows a combination leakage detector and signal level monitor.
The Orndorff device includes two separate input circuits, each including at least one frequency conversion stage. One input circuit is dedicated to leakage signal detection, and the other dedicated to signal level monitoring. The leakage detection input circuit includes one frequency conversion stage, while the signal level monitoring input circuit includes two frequency conversion stages. The two input circuits are then coupled to an intermediate frequency ("IF") section that further includes another frequency conversion stage. The output of the IF frequency conversion stage is a measurement circuit that performs the signal level measurement and leakage detection on the down-converted signal.
The Orndorff device has several drawbacks that impact the cost of manufacturing the product. Specifically, each frequency conversion includes elements that increase the cost of the product. The Orndorff design requires four frequency conversion stages, each of which increases the product cost. Moreover, the Orndorff design as disclosed includes an integrated circuit that houses a plurality of log amplifiers for providing gain control of the frequency converted output signal, which represents another significant portion of the cost. Finally, the Orndorff design employs crystal and ceramic filters that further increase the cost of the product.
There is a need, therefore, for a combined leakage detector and signal level monitor having reduced cost and simplified circuitry.